This invention relates to circuit breakers in general, and in particular to an improved spiral arc circuit breaker which produces a more effective arc-extinguishing action and which is of a simpler structure than presently-existing spiral arc circuit breakers.
At present, the most commonly used type of circuit breaker for handling high voltages is the puffer-type circuit breaker in which a cooling gas is forcefully blown by a piston against an arc produced at the time of breaking, thereby extinguishing the arc. The puffer-type circuit breaker has the disadvantage that a driving mechanism is necessary to drive the piston which blows the cooling gas against the arc, and accordingly a small-sized puffer-type circuit breaker can not be achieved.
To overcome this and other disadvantages of the puffer-type circuit breaker, the rotary arc circuit breaker and the spiral arc circuit breaker were developed. In these devices, a magnetic force is applied to the arc produced between open contacts at the time of circuit breaking, causing the arc to either rotate or spiral at a high speed between the open contacts. The motion of the rotating or spiraling arc relative to a stationary cooling medium such as SF.sub.6 gas contained within the circuit breaker cools the arc. At the same time, the rotation or spiraling lengthens the arc to the point where the system voltage can no longer sustain the arc, and the arc is thereby extinguished. Since it is not necessary to employ mechanical means to blow the gas against the arc, the structure of a rotary arc or spiral arc circuit breaker is simpler than that of a puffer-type circuit breaker, and a circuit breaker of smaller size is achievable.
FIGS. 1 and 2 show longitudinal cross sections of a conventional rotary arc circuit breaker and a conventional spiral arc circuit breaker, respectively. In the figures, element number 1 is a fixed electrode, element number 2 is a movable electrode which slides into and out of contact with the fixed electrode 1, element number 3 is an arc runner, element number 4 is a magnetic drive coil, element number 5 is an arc which develops between the fixed electrode 1 and the movable electrode 2 when the circuit breaker opens, and element number 6 is an electrically insulating member which protects the movable electrode 2 of the spiral arc circuit breaker in FIG. 2.
The operation of these conventional circuit breakers is briefly as follows. At the time of circuit breaking, the movable electrode 2 is separated from the fixed electrode 1, and an arc 5 carrying a current i develops between the movable electrode 2 and the fixed electrode 1. The arc 5 then shifts from the fixed electrode 1 to the arc runner 3. Both circuit breakers are so designed that the current i also flows through the magnetic drive coil 4, which produces a magnetic flux phi. This flux acts upon the arc 5 and causes the arc to either rotate (in the circuit breaker of FIG. 1) or spiral (in the circuit breaker of FIG. 2) at a high speed between the movable electrode 2 and the arc runner 3. The rotating or spiraling arc 5 is extinguished by the cooling effect of the relative motion between it and a stationary cooling gas such as SF.sub.6 contained in the circuit breaker, and by the lengthening of the arc 5 due to rotation or spiraling.
The rotary arc circuit breaker of FIG. 1 has the disadvantage that the flux phi in the vicinity of the movable electrode 2 grows weaker as the distance between the movable electrode 2 and the arc runner 3 increases. When the separation between the two is large, the force produced by the flux phi on the arc 5 in the vicinity of the movable electrode 2 is much weaker than the force acting on the portion of the arc 5 in the vicinity of the arc runner 3. As a result, the force does not produce adequate rotational movement of the arc 5 and adequate cooling and lengthening of the arc 5 can not be achieved.
On the other hand, the spiral arc circuit breaker illustrated in FIG. 2 has the disadvantage that it is difficult to make the arc 5 and the flux phi intersect at right angles. In order for the flux phi produced by the magnetic drive coil 4 to effectively exert force on the arc 5, the directions of the arc 5 and the flux phi should be as nearly perpendicular to one another as possible, since no force acts on the arc 5 when it is parallel to the flux phi. In the spiral arc circuit breaker of FIG. 2, it is necessary for the movable electrode 2 to have an E-shaped cross section so that the direction of the current i flowing through the bottom portion 2a of the movable electrode 2 will be opposite to the direction of the current i flowing through the bottom portion 3a of the arc runner 3. The magnetic forces produced by these currents i flowing in opposite directions react to cause the arc 5 to bulge outwards towards the magnetic drive coil 4 in its midportion 5a, and portions of the arc 5 are thereby able to intersect the flux phi at right angles.
As can be seen from FIG. 3 which shows a view of a portion of the circuit breaker of FIG. 2 when the separation between the movable electrode 2 and the arc runner 3 is small, this bulge in the midportion 5a of the arc 5 causes the arc 5 to intersect the flux phi at right angles in the portion 5b near the arc runner 3 and in the portion 5c near the end of the movable electrode 2, and the arc 5 is caused to spiral. However, when the separation increases to that shown in FIG. 4, the above-described reaction between the current i in the bottom portion 2a of the movable electrode 2 and the current i in the bottom portion 3a of the arc runner 3 is no longer effective in the vicinity of the arc runner 3. A bulge in the arc 5 resulting from the E-shape of the movable electrode 2 is produced in the vicinity of the movable electrode 2, but in the vicinity of the arc runner 3, the arc 5 is parallel to the flux phi and no force acts upon it. In this case, the arc 5 will not spiral and the desired cooling and lengthening of the arc 5 can not be produced.
The above-described drawbacks of rotary arc and spiral arc circuit breakers have made the development of high voltage, large-capacity circuit breakers of this kind difficult.